Thermoplastic polyamide compositions are desirable for use in a wide range of applications including parts used in automobiles, electrical/electronic parts, household appliances and furniture because of their good mechanical properties, heat resistance, impact resistance and chemical resistance and because they may be conveniently and flexibly molded into a variety of articles of varying degrees of complexity and intricacy.
As an example, thermoplastic polyamide compositions are particularly suited for making housings for hand held electronic devices, such as mobile telephones, personal digital assistants, laptop computers, tablet computers, global positioning system receivers, portable games, radios, cameras and camera accessories. Such applications are highly demanding applications since they require polyamide compositions that exhibit a good balance of mechanical properties and aesthetical aspect (e.g. surface appearance and low sink marks) while not interfering with the intended operability of the hand held electronic device, e.g. through absorption of electromagnetic waves.
In an attempt to improve mechanical properties, it has been the conventional practice to add various reinforcements to the resin, like for example glass fibers, glass flakes, carbon fiber, mica, wollastonite, talc, calcium carbonate so as to obtained reinforced polyamide compositions. Glass fibers are known to offer excellent dispersion in thermoplastic polymers and lead to good mechanical properties under standard conditions. Moreover, it is important that the housings of such devices made from polyamide compositions be able to withstand the rigors of frequent use. It is often desirable that such compositions have good stiffness and impact resistance and that they exhibit excellent surface appearance.
Semi-crystalline nylons such us polyamide 66 are known to provide excellent mechanical properties. Nevertheless, articles molded from polyamide 66, for example, exhibit significant changes in mechanical properties upon moisture absorption and therefore are not suitable for such applications. Moreover, semi-crystalline polymers exhibit shrinkage during crystallization in the mold. This may lead to surface defects known as sink marks in certain areas of molded components. Typically sink marks appear in the area of locally higher thickness which often is on the opposite side of reinforcing ribs. Sink marks are depressions or dimple indentations on the surface of injection molded plastic parts thus leading to a poor surface quality of the molded part. Sink marks are imperfections that appear as dimples, craters or ripples. Such defects are undesirable and not acceptable because of an accompanying reduction of the aesthetic surface appearance and because sink marks remain visible after painting the article. In an attempt to reduce sink marks on molded parts, there have been proposed to use or add amorphous polyamides. Nevertheless, amorphous nylons have usually lower mechanical properties, for instance they are more brittle. Melt blending amorphous polyamides with comparable or lower amount of semi-crystalline polymers is a known way to improve mechanical properties. However such blends do not simultaneously yield satisfactory mechanical properties and surface appearance because they result either in poor surface appearance or in articles having too high sink marks.
US 2008/0167415 discloses reinforced polyamide molding materials comprising an aliphatic partly crystalline polyamide and flat glass fibers with elongated shape. Disclosed examples comprise 50 wt-% of a blend of an amorphous polyamide (PA 6I/6T) and an aliphatic partly crystalline polyamide (PA66) and 50 wt-% of flat glass fibers. Whereas this composition exhibits good mechanical properties and a good surface appearance in terms of gloss, this composition exhibits deep sink marks.
Unfortunately, existing technologies fail to combine good mechanical properties and a high quality aesthetical aspect in terms of surface appearance and low sink marks.
A need remains for reinforced polyamide compositions that have a good balance of properties in terms of surface appearance, low sink mark formation and good mechanical properties.